1. Field of the Invention
This invention relates to a method of forming color images by imagewise modulation of the Christiansen effect.
2. Description of the Prior Art
It is well known that, in general, a train of light waves changes direction, i.e., is refracted, when it crosses a boundary separating two media of different indices of refraction and that, except in special cases, only a portion of the incident light passes into the second medium, the remainder being reflected. The directions of propagation of both the reflected and transmitted waves are different from that of the incident wave. These phenomena are exploited in the Christiansen filter which produces a narrow band-pass of color and which has application in the infrared and ultraviolet as well as in the visible region. This filter was first described by C. Christiansen in 1884. The Christiansen filter consists of a finely divided transparent material suspended in an optically homogeneous medium; the constituents are chosen so that they have different but intersecting dispersion curves, i.e. the refractive indices are identical at a particular wave length .lambda..sub.c in or near the visible region, but differ for all other wave lengths. The filter is optically homogeneous for light of wave length .lambda..sub.c, i.e., such light is unaffected and passes through the filter without deviation or reflection. However, for all other wave lengths the filter is optically heterogeneous and such light is scattered as a result of the refraction and reflection which occur at the particle-medium interfaces. The degree of scattering for a given wave length depends on the difference in the two refractive indices at that wave length so that greater scattering is expected for wave lengths farther from .lambda..sub.c. Consequently, the transmission curve for the unscattered light exhibits a maximum at .lambda..sub.c.
Several applications based on the Christiansen filter effect are known. Fields, U.S. Pat. No. 3,586,417 describes a variable color filter. A change in temperature results in different shifts in the dispersion curves of the two constituents thereby changing .lambda..sub.c and the color transmitted. One of the references given in the Fields patent is E. D. McAlister, Smithsonian Misc. Collections 93, No. 7 (1935) which describes such temperature-tunable filters made in the 1920's and 1930's. Matovich, I.S.A. Journal 12, 53 (1965) describes a variable color filter in which the change in .lambda..sub.c is achieved by applying external pressure thereby shifting the dispersion curve of the liquid. George U.S. Pat. No. 3,458,249 describes an optical filter which has a notched region (a high absorption region) within the pass band so that light corresponding to the wave length of the notch is not transmitted but light of higher and lower wave length within the pass band is transmitted. Barnes and Bonner, Phys. Rev. 49, 732 (1936) describe infrared Christiansen filters and the above McAlister reference discusses the ultraviolet filters reported by von Fragstein in 1932-1933. The Christiansen effect is responsible for the colors in the chromatic emulsions of the type prepared by Holmes and Cameron (J. Am. Chem. Soc. 44, 71 (1922)).
No prior art exists that discloses the process of forming color images by imagewise modulation of the Christiansen effect. Strong (Concepts of Classical Optics, Freeman, San Francisco 1958, p. 583) states that the Christiansen filter "cannot be used in an image-forming system --e.g. in front of a camera lens." The Christiansen cell is considered by Strong and others as a filter; there is no suggestion of storing information either temporarily or permanently by adjusting the properties of the Christiansen cell itself which when illuminated results in the desired color image.
In somewhat related work, Taylor (Proc. of the IEEE 61, 148 (1973)) describes a reflective liquid-vapor display principle involving vaporization of a film of transparent liquid from a roughened glass surface. The liquid, glass, and transparent electrodes needed to heat the liquid are chosen with matching indices of refraction so that when the liquid wets the glass, the system is transparent and the viewer sees through to the back wall of the cell which may be black or a color. When the liquid is evaporated so that a vapor (of different index of refraction) forms around the roughened surface, scattering occurs and the viewer sees the translucent whitish appearance of roughened glass. An off-condition color can also be obtained by dyeing the liquid the appropriate color. The display consists of the whitish image on a uniform black or color background. This method of display does not utilize the Christiansen effect.